This invention relates to hydraulic power units of the type which incorporate a cylinder block containing reciprocating pistons which, through pivotally attached slippers, are in operational engagement with a swashplate. Such hydraulic power units may operate as either pumps or motors. Specifically this invention is concerned with an improved piston for use in such hydraulic power units.
Typically, the pistons that are utilized in hydraulic power units of the aforementioned type have been manufactured from a metallic material, usually steel, which is heat treated. The particular material and heat treatment process utilized are selected to provide adequate structural strength and wear properties necessary for appropriate piston function and life. Closed end hollow pistons are shown in U.S. Pat. No. 3,319,575. Fluid conduits extending the length of the piston are shown in both the U.S. Pat. No. 3,319,575 and also in U.S. Pat. No. 3,882,762.
Because the piston interacts dynamically with both the cylinder block and the slipper-to-swashplate connection which present different loading and wear challenges to the piston, it has been determined that the material property requirements of the piston are not necessarily uniform throughout the entire structure of the piston. For this reason, it is advantageous to provide a piston which is made up of more than a single material so that various structural elements of the piston may possess different strength and wear properties in its final form.
Generally, it has been determined that the cylindrical body of the piston which is in reciprocating contact with the bore of the cylinder block requires strength and surface wear properties necessitating the use of hardened medium carbon steel. The end of the piston which is connected to a slipper through a spherical joint is subject to compressive rotary and pivotal motion and does not require the extreme strength and wear properties of the cylindrical body. The spherical end of the piston can provide satisfactory function and life when formed of low carbon steel which is not hardened. Additionally, it is often advantageous for the spherical end of the piston to remain malleable to crimping of the end to retain the slipper to the piston in an assembled condition.
Therefore, it is a principal objective of this invention to provide a piston which is comprised of more than a single material.
Further, it is an objective of this invention to form a piston from multiple parts, each part being of a different metallic material.
It is also an objective of this invention to produce a piston which includes a hardened cylindrical body and a malleable spherical end.
It is a further objective of this invention to provide a piston which incorporates a conduit traversing the body and the insert as a path to communicate pressurized fluid from the cylinder bore to the slipper and swashplate operative connection.
Additionally, it is an objective of this invention to assemble the piston to a slipper for subsequent reciprocal operation in the bore of a cylinder block and operative connection with a swashplate in a hydraulic power unit.
These and other objectives will be apparent to those skilled in the art.